In multi-cavity stack injection molding systems, accurate control of resin melt flow from a hot runner manifold to all of the mold cavities is essential to achieving uniformity of the molded articles. The reciprocating valve pins used to control the gates to the cavities typically are operated simultaneously, which requires even loading of the valve pins. Compactness and simplicity of the machinery also are important considerations from the standpoint of cost, reliability and efficient heat transfer.
Representative examples of known stack injection molding systems are disclosed in U.S. Pat. Nos. 4,212,627; 4,244,909; 4,891,001; 5,013,235; 5,460,510; 5,478,230; and 5,533,882. Some of these employ a separate actuator for each valve pin, or a separate actuator for each opposed pair of valve pins. U.S. Pat. No. 4,212,627 discloses a stack molding arrangement wherein a single hydraulic actuator simultaneously moves all of the valve pins of two arrays of injection nozzles through a complex multi-link slide bar mechanism. This arrangement ensures simultaneous valve pin movement, but it tends to require frequent maintenance and does not permit any flexibility in operation, such as differential opening or closing of the two arrays of injection nozzles.
In single-face molding machines various arrangements are known for simultaneously actuating a single array of valve pins. See, for example, U.S. Pat. No. 4,095,931 (sliding cam rod); U.S. Pat. No. 4,330,258 and WO 00/43187 (rack and pinion drive); and JP 9141688 (reciprocating plate). Other valve pin actuating mechanisms are disclosed in U.S. Pat. No. 4,378,963; U.S. Pat. No. 4,669,971; U.S. Pat. No. 4,712,995; U.S. Pat. No. 4,917,594; U.S. Pat. No. 4,923,387; U.S. Pat. No. 5,368,470; DE 3733363; DE 4230758; and DE 4231270. However, the technology used in such arrangements has not been of much benefit to stack molding apparatus, which generally tend to be complex, cumbersome, expensive, subject to premature wear, and/or unreliable. A need exists, therefore, for an improved stack molding apparatus that is durable and compact, allows for flexibility of operation, and efficiently and reliably produces molded products of good quality.